1. Field of the Invention
The present invention generally relates to a method and apparatus for metal melting, refining and processing, for example steelmaking in an electric arc furnace (EAF), and more particularly, to an auxiliary burner or lance for the injection of either additional thermal energy, oxidizing gas for oxidizing liquid metal, post combustion of carbon monoxide or the like, and particulates for slag and foamy slag production, or the like.
2. Description of Background Art
Oxygen and carbon injection lances are known in the art of steelmaking to be useful for the injection of these materials or others to enhance many steps in the process. In addition, oxy-fuel burners have been used to provide auxiliary thermal energy and supersonic oxygen to these processes. Additionally, there have been some attempts to combine oxygen and carbon injection lances with the oxy-fuel burner function. An important question for the integration of these functions into one apparatus has been whether to retain particulate injection capability or supersonic oxygen capability because both functions are the most advantageous if located along the central axis of the lance or burner.
Particulate injection is best done through a straight conduit which is located along the central axis of the apparatus used. A straight conduit is conventional because the particulates injected into a steel making furnace are highly abrasive and will wear out bends or other restrictions to their flow quickly. This is one of the reasons why particulates have not be injected through the same conduit as the one used for supersonic oxygen of a burner, the particulates would quickly wear out the converging restriction of the nozzle. A central conduit is preferred because it is highly disadvantageous to break the stream into more than one flow because one would like to concentrate particulates in a specific area. Also, the size of the particles and amount of particulates used for an injection is large in mass compared to other injected materials, such as gases, and a relatively large conduit is needed for reasonable flow rates.
Laval or supersonic nozzles are usually used in the production of high speed streams of oxidizing gas for injection into a steel making furnace. These supersonic gas flows are produced by the converging/diverging shape of the nozzle which at above a critical pressure causes the gas flow though the nozzle to become supersonic. Usually, an conduit is machined centrally in a lance or burner and then the passage is fitted with a converging/diverging section or nozzle. A large centrally located nozzle is desired because of the flow rates of supersonic oxygen desired.
It is also highly desirable to provide a subsonic flow of oxidizing gas for the burning of fuel for the addition of auxiliary thermal energy, and the supersonic oxygen flow for providing oxygen in iron melt decarburization, assisting in foamy slag production or post combustion of carbon monoxide. A burner which provides subsonic and supersonic flows of oxygen through the same centrally located conduit is manufactured and commercially sold by Process Technology International, Inc. of Tucker, Georgia. The subsonic flow is produced by providing a pressure in the supply conduit lower than the critical pressure of the Laval nozzle being used in the conduit. When supersonic oxygen is needed the pressure in the supply conduit is increased to above the critical pressure.
One attempt to combine the functions of carbon injection and supersonic oxygen in one apparatus is shown in U.S. Pat. No. 5,599,375. In FIGS. 3 and 5, a burner is described having carbon injection and supersonic oxygen. However, the carbon injection is not coaxial to the stream of oxidizing gas introduced through the burner and cannot be directed in sufficient quantities to be advantageous. Another attempt is shown in the same reference in FIG. 6 where a central carbon injection pipe is surrounded by a plurality of oxygen generating apertures which are described as Laval nozzles. This configuration is highly disadvantageous due to the small supersonic openings and dispersion of the supersonic oxygen due to flow turbulence of each small aperture interacting with that of the other apertures.
The invention provides an improved method and apparatus for steelmaking. The method includes the steps of providing additional thermal energy to the steel making process, providing particulate injection for the formation of foamy slag, and providing oxidizing gas injection for the decarburization of the melt, formation of foamy slag and post combustion of CO. These steps may be accomplished in any order, and may be accomplished either alone or in combination with one or more of the other steps. In addition, the step of providing oxidizing gas may provide it at high velocity which preferably is supersonic or at a lower velocity such as subsonic.
The apparatus provides a unique burner configuration that in a single integrated apparatus can efficiently perform the multiple functions of the method. The burner accomplishes this by operating in multiple modes including at least a burner mode, an oxygen lancing mode and particulate injection mode.
The invention in the apparatus implementation includes a unique burner configuration which has a central conduit for selectively supplying either fluid hydrocarbon fuel or particulate matter, preferably carbon particles, which is entrained in a carrier or transport gas through its exit opening. The fuel or carbon particles are mixed with a high speed annular stream of gas, preferably an oxidizing gas such as commercially pure oxygen. In the preferred embodiment, the high speed flow of oxidizing gas is provided by an annular supersonic nozzle which causes an annular flow of oxidizing gas to selectively surround the fuel or the carbon particles. The annular nozzle allows a coaxial annular flow of oxidizing gas to be mixed with the fuel or particulates at subsonic or supersonic rates, while still being able to supply independent supersonic oxidizing gas with a desirable lancing capability at other times.
Several implementations of the annular nozzle are shown which are used to direct the flow of the oxidizing gas, and as a consequence the fuel or the particulates, in a desired pattern for performing a specialized function. The annular flow from the nozzle can be tailored from a substantially inwardly directed flow where the annular flow tends to concentrate toward the center axis of the nozzle to a substantially outwardly directed flow where the annular flow tends to disperse from the center axis of the nozzle, to anywhere in between, such as a partially inwardly directed flow and partially outwardly directed flow. The shaping of the annular flow is accomplished by varying the contour that the inner and outer surfaces which form the annular diverging section of the nozzle make with the centerline of the annular restriction. This variation in the shaping of the diverging section essentially redirects the annular flow vector from parallel to the central axis of the nozzle to either inwardly toward the central axis or outwardly from the central axis, or any combination therebetween.
In one advantageous implementation, the diverging section of the annular nozzle has an asymmetric cross-section where the outer surface contour diverges away from the nozzle center axis more quickly than the inner surface contour. This embodiment tends to cause the annular flow vector to be outwardly directed from the center axis of the nozzle. Another advantageous implementation has an asymmetric cross-section with the inner surface contour of the diverging section of the annular nozzle diverging from the center axis of the nozzle more quickly than the outer surface contour. This embodiment tends to cause the annular flow vector to be inwardly directed toward the center axis of the nozzle. Still another advantageous implementation has an symmetric cross-section with the inner surface contour of the diverging section of the annular nozzle diverging from the center axis of the nozzle at substantially the same rate as the outer surface contour. This embodiment tends to cause the annular flow to be equally inwardly directed toward the center axis of the nozzle and outwardly directed from the center axis of the nozzle.
In an optional embodiment, the annular supersonic nozzle in any of its various embodiments is surrounded by a plurality of shrouding apertures which are supplied with a pressurized gas to produce a subsonic but high velocity shroud of gas surrounding the annular supersonic flow. Preferably, but not necessarily, the shrouding gas is an oxidizing gas. If the shrouding gas is an oxidizing gas, preferably it supplied from the same source as supplies the annular nozzle.
Optionally, in another embodiment the burner has another conduit for the supply of a pressurized flow of a second fuel flow surrounding the annular flow of oxidizing gas from a series of first apertures.
Optionally, in yet another embodiment the burner has another conduit for the supply of a pressurized flow of a second oxidizing gas surrounding either the annular flow or secondary fuel flow from a series of second apertures.
These and other objects, aspects and features of the invention will be more clearly understood and better described when the following detailed description is read in conjunction with the attached drawings, wherein: